First attested early 1400s as various Middle English forms prayne, prane, praune, and prawne, which present no clear cognates in languages other than English. The forms suggest a hypothetical Old English form *prægn, where *æg would have evolved into Middle English *ay, but it is unclear if the word is of Germanic origin, from another European language, or loaned from a substrate. In the Isle of Wight, a word prankle ("prawn") is recorded and thought to be related. Century, following Skeat, suggested transposition of an unrecorded Old French *parne, *perne related to Spanish perna (“a flat shellfish”), Old Italian perna and diminutive pernochie, parnocchie, glossed as "shrimps or prawne, fishes" by John Florio, but the OED considers Florio's entry incorrect and the suggested connection semantically and phonologically implausible. Etymology 1, noun sense 3 ("woman with attractive body and unattractive face") is from the idea of discarding the head of a prawn before eating it.
1
A crustacean of the suborder Dendrobranchiata.
The aphrodisiac nature of prawns is evident in an epigram by the Greek poet Asclepiades.1961, Harry E. Wedeck, Dictionary of Aphrodisiacs, New York: The Citadel Press, page 197:
2
Commonwealth A crustacean, sometimes confused with shrimp.
This is utter dribble. I've not read much worse than this in a long time - and he admits he doesn't know - "or so I am told". Get real, you prawn.1999 August 2, Les Brown, “This old "almah" controversy”, in alt.religion.christian (Usenet):
He didn't say he was accused of that, prawn.2001 February 1, Ned Latham, “Lovesick Puppy Poetry - Volume 1”, in aus.culture.true-blue (Usenet):
First attested early 1400s as various Middle English forms prayne, prane, praune, and prawne, which present no clear cognates in languages other than English. The forms suggest a hypothetical Old English form *prægn, where *æg would have evolved into Middle English *ay, but it is unclear if the word is of Germanic origin, from another European language, or loaned from a substrate. In the Isle of Wight, a word prankle ("prawn") is recorded and thought to be related. Century, following Skeat, suggested transposition of an unrecorded Old French *parne, *perne related to Spanish perna (“a flat shellfish”), Old Italian perna and diminutive pernochie, parnocchie, glossed as "shrimps or prawne, fishes" by John Florio, but the OED considers Florio's entry incorrect and the suggested connection semantically and phonologically implausible. Etymology 1, noun sense 3 ("woman with attractive body and unattractive face") is from the idea of discarding the head of a prawn before eating it.